This invention relates generally to telephone conferencing circuits, and more particularly to telephone conferencing circuits employing digital techniques and wherein each participant (conferee) receives the digital signals from the two loudest of all the other participants (conferees).
Conferencing circuits are well known in the field of telephony. In general terms, a conference circuit is a circuit for allowing three or more participants (or conferees) to talk to one another at the same time. Early conference circuits, employed in analogue telephone systems, provided conferencing by summing all the signals of all the participants and transmitting this resultant signal to all the conferees, with the exception of the talker who received the resultant signal minus his own signal. As telephone technology advanced into the world of digital techniques, simple summing and subtracting no longer provided an easy solution to the problem of conferencing.
Some prior art approaches to conferencing with digital techniques were simply to convert the digital signals to analogue signals, perform an analogue conferencing, and re-convert the resultant analogue conference signal into a digital signal. One example of such an approach is shown in U.S. Pat. No. 3,970,797 dated July 20, 1976 to D. A. Johnson and Wm. C. Towle. It is, however, cumbersome to conference in this manner if it is possible to conference directly in digital format. Additionally, the converting to analogue and reconverting to digital adds distortion to the signals involved.
An improvement over the analogue summing of signals for conferencing is to do the summing directly with digital signals. Since the digital signals are commonly not linear, but rather are non-linearly Pulse Code Modulated (PCM), it is necessary to first linearize the digital signals, add them, and then re-code (all the while remaining in the digital domain). U.S. Pat. No. 3,924,082 dated Dec. 2, 1975 to S. E. Oliver and N. R. Winch and U.S. Pat. No. 4,190,744 dated Feb. 26, 1980 to R. J. Frank describe two such systems.
A further modification in conferencing circuits is to provide a digital conferencing circuit which performs the conferencing function directly using the coded digital signals. U.S. Pat. No. 4,007,338 dated Feb. 8, 1977 to D. W. McLaughlin, U.S. Pat. No. 4,031,328 dated June 21, 1977 to S. G. Pitroda, and U.S. Pat. No. 4,224,688 dated Sept. 23, 1980 to C. A. Ciancibello and E. A. Munter, to mention just a few, all depict such a conferencing circuit. In circuits of this type each conferee receives the one PCM word judged the largest (i.e. loudest) from all the other conferees. As an example in a three party conference, two PCM words (corresponding to two time slots or channels) are compared and the largest PCM word is transmitted to the third channel.